This invention relates generally to artificial ground freezing and more particularly to an improved process and system that has particular utility in large scale ground freezing applications.
Artificial ground freezing has been used for many years to freeze selected areas of the ground for a number of different purposes. It is often used to provide support for excavations or to cut off ground water seepage, although it can be used for applications such as the confinement of hazardous materials in the ground and creating impermeable zones for hydrocarbon or mineral extraction or processing.
When the soil is frozen, the water within the soil freezes and bonds the soil particles together. It has been determined that colder soil temperature significantly increase the strength of the frozen soil, to the point where its compressive strength can equal that of some types of concrete. The combination of high strength and impermeability makes frozen soil useful as a shoring system for deep excavations. By way of example, mine shafts well over 1000 feet deep have been completed using ground freeze shoring techniques. Frozen soil walls for preventing ground water or chemicals in the soil from migrating through the ground have been formed to provide a barrier in cases where there is no need for an open excavation such as a mine or tunnel.
In a conventional ground freeze application, drilling is carried out to form spaced apart bores in which freeze pipes are installed around the perimeter of the proposed excavation or along the ground water barrier. Typically, the freeze pipes are steel pipes three to four inches in diameter installed three to six feet apart along the site of the proposed wall of frozen soil. The most commonly used technique involves circulating a refrigerated liquid through the freeze pipes. Salt water brine and ethylene glycol can be used, and they are cooled using a vapor compression cycle refrigeration system that employs a refrigerant such as ammonia, R-22 or other refrigerant. The refrigeration plant is specially designed for ground freezing and may be either mobile or stationary. After the circulating fluid has been chilled, it is pumped through the freeze pipes and is returned to be cooled again by the refrigeration plant. The entire system is closed to the atmosphere.
As the cold liquid circulates through the freeze pipes, the soil around each individual pipe freezes. As more time passes and more circulating liquid is pumped through the freeze pipes, the frozen zone of soil around each pipe is enlarged until the adjacent zones eventually merge to form a barrier that is impermeable. As the freezing process continues and additional freezing occurs, the frozen barrier increases in thickness and the temperature decreases. The result is that a continuous barrier is created so that excavation can take place or, in the case of a ground water barrier, a containment wall is formed.
Another ground freezing technique that has been used is known as a direct expansion process in which a cryogenic fluid such as liquid nitrogen or liquid carbon dioxide is applied to the freeze pipes. The fluid boils to a vapor to extract heat from the soil and then discharges to the atmosphere. In an open system of this type, the fluid is not recirculated but is essentially lost to the atmosphere. The advantage of the direct expansion system is that it freezes the ground much faster than a brine circulation system. However, the cryogenic fluids are so costly that it is not practical to use them in many applications and particularly in large scale projects.
Each ground freezing project requires an evaluation to determine the appropriate spacing between the freeze pipes. Increasing the spacing between pipes results in a longer time required for the ground to be frozen to form the barrier. Three to six weeks of freeze time is typical for the freeze zone to be completed with the necessary permeability. The time can be reduced by either using a colder circulating fluid or by reducing the pipe spacing. However, if the pipe spacing is reduced, more drilling is required. Because drilling is the single most costly aspect of a ground freezing project, it is highly undesirable to space the pipes close together. Conversely, the overall cost can be reduced significantly by increasing the pipe spacing to decrease the drilling requirements. With increased distance between pipes, the only way for an effective frozen barrier to be formed in a reasonable time period is to decrease the temperature of the coolant that is circulated through the freeze pipes.
On some projects, coolant temperatures must be about xe2x88x9252xc2x0 C. (xe2x88x9262xc2x0 F.) or less to allow a pipe spacing that is consistent with a reasonably low drilling cost. However, conventional circulating fluids such as calcium chloride brine or ethylene glycol cannot attain such a low temperature.
The present invention is directed to a method and apparatus for ground freezing that makes use of cooling techniques resulting in circulating fluid temperature of xe2x88x9250xc2x0 C. (xe2x88x9258xc2x0 F.) or less. This has the great advantage of allowing the freeze pipes to be spaced relatively far apart while still creating an impermeable frozen earth barrier in a reasonable period of time. The saving in drilling cost that results from the need for fewer freeze pipe bores creates a major economic benefit making ground freezing practical for very large projects.
In accordance with the invention, a refrigeration system is used to cool a circulating heat transfer fluid to a temperature of xe2x88x9252xc2x0 C. (xe2x88x9262xc2x0 F.) or less. The heat transfer circulating fluid is preferably aqua ammonia (ammonium hydroxide) with 27-30% ammonia, which has the advantage of being readily available at a low cost and the ability to serve as an efficient heat transfer fluid. Equally important, aqua ammonia (ammonium hydroxide) has a very low viscosity (actually less than water at xe2x88x9252xc2x0 C.) so that it can be easily pumped through the freeze pipes to minimize pumping costs and difficulties.
The refrigeration plant used to cool the circulating fluid may advantageously employ low and high stage vapor compression refrigeration systems arranged in a cascade relationship with one another. The low stage system may use carbon dioxide as its refrigerant with its condenser arranged to discharge its heat to the evaporator of the high stage system. Ammonia is preferably the refrigerant in the high temperature system. However, R-22 or other refrigerant may be employed. In this way, the low temperature system can cool the circulated fluid to the requisite temperature xe2x88x9252xc2x0 C. (xe2x88x9262xc2x0 F.) or less and thus allow the freeze pipes to be spaced relatively far apart far so that the drilling costs are low enough to make ground freezing practical in large scale projects.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.